1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a step-down converter and a power supply, and more particularly, to a step-down converter and a power supply which can be stably operated at a time of an initial start up by outputting a voltage lower than an input voltage.
2. Description of the Related Art
In general, a power supply converts electric power supplied from an external source in accordance with a demand of an internal system and supplies the converted electric power. An example of such a power supply includes a switched-mode power supply.
The switched-mode power supply controls the flow of electric power using a switching process of a semiconductor element. Because the switched-mode power supply has high efficiency, small size, and light weight, it is widely used for electric, electronic, and communication apparatuses.
A DC-DC converter is an example of the switched-mode power supply. The DC-DC converter converts a direct current (DC) voltage from one level to another level. There are various types of DC-DC converters for outputting a desired voltage based on an input voltage, such as, for example, a buck or step-down converter, a boost converter, and a fly back converter.
The step-down converter outputs an output voltage lower than an input voltage. The step-down converter has a switching element and an energy storage element to perform switching operation and generate an output voltage lower than an input voltage. An N-channel Metal Oxide Semiconductor (NMOS) transistor may be used as a switching element of the step-down converter because of the ease of integration and low price.
In order to perform the switching operation, the NMOS transistor should have a gate voltage Vgs greater than a threshold voltage. Therefore, the step-down converter using the NMOS transistor employs a boost capacitor and uses a voltage charged in the boost capacitor in the switching operation as a gate driver power of the NMOS transistor.
One drawback of a related art step-down converter is that an input voltage may become lower than an output voltage depending on the load conditions of an input and an output after completing the converting operation.
FIG. 5A illustrates a waveform diagram of an input voltage Vin and an output voltage Vout when the electric power is interrupted in a related art step-down converter. As shown in FIG. 5A, if the electric discharge of an input is faster than that of an output, the output voltage becomes greater than the input voltage after a time T1. At this time, if the step-down converter begins to operate, the boost capacitor cannot perform charging operation in the state that the output voltage is greater than the input voltage. Accordingly, the operation of the step-down converter may result in a fault.